Phase shifting interferometer

ABSTRACT

An interferometer which has the capability of measuring optical elements and systems with an accuracy of λ/1000 where λ is the wavelength of visible light. Whereas current interferometers employ a reference surface, which inherently limits the accuracy of the measurement to about λ/50, this interferometer uses an essentially perfect spherical reference wavefront generated by the fundamental process of diffraction. Whereas current interferometers illuminate the optic to be tested with an aberrated wavefront which also limits the accuracy of the measurement, this interferometer uses an essentially perfect spherical measurement wavefront generated by the fundamental process of diffraction. This interferometer is adjustable to give unity fringe visibility, which maximizes the signal-to-noise, and has the means to introduce a controlled prescribed relative phase shift between the reference wavefront and the wavefront from the optics under test, which permits analysis of the interference fringe pattern using standard phase extraction algorithms.

The United States Government has rights in this invention pursuant toContract No. W-7405-ENG-48 between the United States Department ofEnergy and the University of California for the operation of LawrenceLivermore National Laboratory.

CROSS REFERENCE TO RELATED APPLICATION

This application is a Continuation of co-pending International Appln. NoPCT/US95/15274, filed on Nov. 21, 1995, which is a Continuation-In-Partof U.S. patent application Ser. No. 08/345,878, filed on Nov. 28, 1994,now U.S. Pat. No. 5,548,403. These prior filings are incorporated byreference herein.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to interferometry, and more specifically,it relates to high accuracy diffraction interferometry.

2. Description of Related Art

Interferometry is the preferred method to measure the performance ofoptical elements and systems. In this method the wavefront of lightreflected from or transmitted by the optic to be tested is interferedwith the wavefront from a reference surface, to produce an interferencefringe pattern. These interference fringes are then analyzed toascertain the performance of the optic. For high performance imagingsystems, such as those found in lithographic steppers used to makeintegrated circuits, this interferometric measurement must be made toever increasing accuracy. The accuracy, however, is limited by how wellthe reference surface is characterized. Reference surfaces are typicallyno better than λ/50, where λ is the wavelength of visible light, andthus are the limiting factor in fabricating higher performance opticalsystems. Therefore the fabrication of high accuracy optical systems,such as those needed for extreme ultraviolet projection lithographywhich require an accuracy of λ/1000, are impossible to qualify withconfidence using existing interferometry.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a phase shiftingdiffraction interferometer having λ/1000 accuracy.

An interferometer is disclosed which has the capability of measuringoptical elements and systems with an accuracy of λ/1000 where λ is thewavelength of visible light. Whereas current interferometers employ areference surface, which inherently limits the accuracy of themeasurement to about λ/50, this interferometer uses an essentiallyperfect spherical reference wavefront generated by the fundamentalprocess of diffraction. This interferometer is adjustable to give unityfringe visibility, which maximizes the signal-to-noise, and has themeans to introduce a controlled prescribed relative phase shift betweenthe reference wavefront and the wavefront from the optics under test,which permits analysis of the interference fringe pattern using standardphase extraction algorithms.

The interferometer described in this disclosure has the ability toachieve this extremely high accuracy by eliminating the referencesurface and substituting an essentially perfect spherical referencewavefront generated by the fundamental process of diffraction. Whereascurrent interferometers illuminate the optic to be tested with anaberrated wavefront which also limits the accuracy of the measurement,this interferometer uses an essentially perfect spherical measurementwavefront generated by the fundamental process of diffraction. Theinvention maximizes the signal-to-noise and permits analysis of theinterference fringe pattern using standard phase extraction algorithms.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an embodiment of the phase shifting diffractioninterferometer.

FIG. 2A shows an embodiment of the interferometer plate shown in FIG. 1.

FIG. 2B shows an embodiment of the interferometer plate shown in FIG. 1.

FIG. 3 shows an embodiment of the interferometer plate shown in FIG. 1.

FIG. 4 shows an embodiment of the phase shifting diffractioninterferometer.

FIG. 5 shows a magnified view of the fiber end shown in FIG. 4.

FIG. 6 shows the elongated end of the fiber shown in FIG. 4.

FIG. 7 shows an embodiment of the phase shifting diffractioninterferometer utilizing coherence length.

FIG. 8 is an embodiment using a single mode optical fiber.

FIG. 9 shows the details at the far end of a single mode fiber.

FIG. 10 is another embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

The interferometer system is shown FIG. 1 in an embodiment that mosteasily illustrates the essence of the idea, but is not necessarily thepreferred embodiment or the only embodiment. It consists of four mainparts: the beam conditioning optics; the interferometer; the detectionsystem; and the computer system.

The major components of the beam conditioning optics are the lightsource, intensity and contrast controls, and the phase shifting module.Referring to FIG. 1, a linearly polarized collimated beam 10 from alight source 12 (e.g., a laser) passes through a variable neutraldensity filter 14 which is used to control the light level to theinterferometer. The beam 10 then passes through a half-wave retardationplate 16 which produces two orthogonally polarized beams, beam 18 havingvertical (out of plane) polarization and beam 20 having horizontal (inplane) polarization, both beams within beam 10. The angular orientationof the half-wave retardation plate 16 is used to adjust the relativeintensity between the vertical and horizontal components ofpolarization. Beams 18 and 20 having these two polarization componentsare then split by a polarization beamsplitter 22 so that the horizontalpolarization (beam 20) is transmitted while the vertical polarization(beam 18) is reflected. The transmitted beam 20 is then reflected by afixed retroreflector 24 back through the polarization beamsplitter 22 toa turning mirror 26. The reflected beam 18 is reflected by aretroreflector 28, mounted on a piezoelectric translator (PZT) 30, backthrough the polarization beamsplitter 22 to the same turning mirror 26so that is it parallel to the other beam but laterally displaced fromit. Applying a voltage to the PZT 30 translates the retroreflector 28thus shifting the relative phase between the two orthogonally polarizedbeams. The two beams pass through a polarizer 32 (axis at 45°) to givethem the same polarization and are brought to focus, with a microscopeobjective 34, on the interferometer plate 36.

Referring to FIG. 2A, the interferometer plate 36 comprises a glasssubstrate 38 coated with a highly reflecting, low transmission, metallicfilm 40, through which a circular aperture 42 has been etched. Thediameter of the aperture 42 is on the order of the wavelength of thesource 12. Metallic film 40 may typically comprise aluminum having athickness of 65 nanometers. In the embodiment shown in FIG. 2B, theinterferometer plate 36 comprises a glass substrate 38 coated with ahighly reflecting, low transmission, metallic film 40, through which acircular aperture 42 has been etched, and over which a second partiallytransmitting metallic film 44, without an aperture, has been coated. Thediameter of the aperture 42 is on the order of the wavelength of thesource 12. Metallic film 40 may typically comprises aluminum having athickness of 65 nanometers and metallic film 44 may typically comprisesaluminum having a thickness of 26 nanometers. In both embodiments of theinterferometer plate 36, the metallic film 40 and the second metallicfilm 44 may comprise materials other than aluminum, for example,chromium, gold, nickel, silicon and silver. The thicknesses of the filmwould vary depending on the material used. This plate serves to generatethe point source measurement beam, using one of the beams focused on theplate, to illuminate the optic under test. It simultaneously serves togenerate the point source reference beam, using the other focused beam.Because the measurement beam and the reference beam are generated bydiffraction, they both comprise a perfect spherical wavefront over somefinite solid angle.

Beams 18 and 20 are focused on the interferometer plate 36 so that theyare both incident on the aperture 42 in the metallic film 40. Both beamsare diffracted by the aperture 42, producing two spherical wavefrontsthat diverge as they leave the aperture. Each wavefront is perfectlyspherical over a finite angular range (defined by the diameter of theaperture relative to the wavelength) centered about the illuminationdirection defined by the lateral separation of the beams as they enteredthe microscope objective. As shown in FIG. 1, measurement beam 46illuminates the optic 50 under test. This optic typically comprises aconcave mirror and is positioned to focus the measurement beam 46 backonto the interferometer plate 36. Due to the finite extent of the optic50 and aberrations therein, the focused measurement beam 46 is muchlarger than the aperture 42 in the metallic film 40, so nearly all ofthe focused measurement beam 46 is reflected by the film 40 in theembodiment of the interferometer plate 36 shown in FIG. 2A. Inembodiment of the interferometer plate 36 shown in FIG. 2B, focusedmeasurement beam 46 is reflected by the film 44. It diverges and iscoincident with the reference beam 48 which was diffracted by theaperture 42. The measurement beam 46 and the reference beam 48 interfereto produce a fringe pattern that represents a contour map of opticalpath difference between the wavefront from the optic 50 and a perfectspherical wavefront.

The detection system consists of an imaging system to image the opticunder test onto a CCD array camera. The imaging system comprises a lens52, an aperture 54 for spatially filtering the interfering beams, and alens 56. The aperture 54 is large enough so that it does not diffractthe beam focused through it. A typical size is 250 micrometers. The sizeof the lenses 52, 56, depends on the size of the optic under test.

The coincident measurement beam 46 and reference beam 48, diverging fromthe intecferometer plate, are collected by a spatial filter imagingsystem which images the surface of the optic 50 under test onto a screen(not shown) or onto a charge coupled device (CCD) camera 58. Thisguarantees that the phase of the interfering wavefronts at each pixel inthe CCD camera 58 has a one-to-one correspondence with a unique point onthe optic 50. It also minimizes effects of edge diffraction from theoptic. Aperture 54, at the intermediate focus of the interfering beams,is used to filter out any light not coming from the immediate areaaround the aperture in the interferometer plate 36. The CCD camera 58captures a series of interference patterns and transfers them to thecomputer system 60.

The computer system consists of a computer having a monitor and softwareto control the light level and contrast of the interference pattern,software to translate the PZT and thus shift the relative phase betweenthe measurement and reference beams, software to calculate the phase ateach pixel using the transferred interference patterns, and software todisplay the resultant phase map. This software for analyzing theinterference pattern read into the computer can presently be supplied byseveral companies. Zygo Corporation produces "Metro Pro" software. PhaseShift Technology produces "Optic Code Analysis Software". WYKOCorporation produces "WISP" software.

The series of interference patterns that are transferred to the computerare captured as the PZT shifts the relative phase of the interferingbeams by 2π radians. The interference patterns are analyzed to determinethe proper settings for the light level and contrast of the interferencefringes. This information is used by the computer to adjust the positionof the neutral density filter 14 and half-wave retardation plate 16. Asecond series of interference patterns are then captured and analyzed todetermine the phase at each pixel. This is typically displayed as acontour or 3D plot of the phase. This phase map corresponds to thedeviation of the surface of the optic from a perfect sphere.

This interferometer is unique in that:

a. The measurement beam is generated by diffraction and is a perfectspherical wave over some finite solid angle. The solid angle is definedby the size of the aperture in the interferometer plate relative to thewavelength of light from the source. Smaller apertures produce largersolid angles.

b. The reference beam is generated by diffraction and is a perfectspherical wave over the same solid angle as the measurement beam.

c. No reference surface is required for this interferometer. Referencesurfaces are a major source of error in interferometry and ultimatelylimit the accuracy that can be achieved.

d. The measurement beam reflected from the optic under test is imagedback onto the aperture of the interferometer plate, giving exactcoincidence with the reference beam. This is the ideal condition forachieving the highest degree of accuracy.

e. The relative phase between the measurement and reference beams can beshifted in a controlled way. This permits a series of interferencepatterns to be analyzed to determine the phase at each pixel positionwith the highest degree of accuracy.

f. The relative intensities of the measurement and reference beams canbe adjusted (with the half-wave retardation plate) to give maximumpossible contrast. This produces the greatest signal-to-noise, necessaryfor achieving the highest degree of accuracy.

Referring to FIG. 3, interferometer plate 37 may be useful in certaincircumstances depending on the optic under test. Here beams 18 and 20each pass through a prism 17, 19 respectively, and are focused by lens34 onto two distinct apertures: aperture 41 and aperture 43 in metallicfilm 45 of the interferometer plate 37. A tilt is introduced betweenbeam 20 and beam 18 by the prisms. The usable metallic films are thesame as described for interferometer plate 36. Apertures 41 and 43 aretypically separated from each other by 10 to 500 micrometers. Thetypical angle between the two focused beams is in the range of 10 to 60degrees. In the embodiment shown on FIG. 3, beam 18 passes through prism17 and is focused by lens 34 onto aperture 43 in interferometer plate37. The resulting diffracted measurement beam 47 is then reflected fromtest optic 51 to the reflective area surrounding aperture 41 ofinterferometer plate 37. This beam is reflected along the same path asreference beam 49 to produce an interference pattern therebetween. Thisembodiment is used to test optical surfaces that are concave withrespect to the interferometer plate 37.

Another distinctive form of this interferometer system, based on fiberoptic technology, operates on the principles described supra. Referringto FIG. 4, light source 12 is focused by lens 62 onto the end of asingle mode fiber 64. A variable fiber coupler 66 splits the beam intotwo fibers: fiber 68 and fiber 72. The ratio of light split into eachfiber is chosen for maximum fringe contrast. Fiber 68 is used for themeasurement beam 70. Fiber 72, transmitting reference beam 74, iswrapped around a PZT cylinder 76 that expands with an applied voltage.This stresses the fiber 72, changing its refractive index, andintroduces a phase shift relative to the measurement beam 70 from fiber68. The measurement beam 70 leaving the end of the fiber is diffracted,producing a perfect spherical wavefront over some finite solid angle.The solid angle is defined by the size of the fiber core relative to thewavelength of light from the source 12. This spherical measurement beam70 illuminates the optic 78 under test, which focuses the beam onto theend of the fiber for the reference beam. FIG. 5 shows a magnified viewof the exit portion of fiber 72. Measurement beam 70 is focused ontoreflective metallic film 73 on the end of fiber 72, and is reflectedalong the same path as reference beam 74. The rest of the system is asin FIG. 1. This embodiment is used to test positive lenses.

Referring to FIG. 6, it may be necessary to shrink the core diameter offibers 68 and 72 to match the aperture size for the interferometer. Thisis accomplished by heating and pulling the fiber. For example, fiber 72,having cladding 84 and core 80 is heated and pulled to produced astretched portion 82 with a core 81 having a diameter that is smallerthan the unstretched core portion 80. Metallic film 73 is coated afterheating and stretching and can comprise any of the films describedsupra. One embodiment of the metallic film comprises aluminum having athickness of 26 nanometers. Single mode fibers typically have corediameters of about 4 micrometers. The measurement beam is reflected fromthe end of the fiber while the reference beam is transmitted through thefilm and diffracted, giving a perfect spherical wavefront over the samefinite solid angle as the measurement wavefront. The solid angle isdefined by the size of the fiber core relative to the wavelength oflight from the source 12. The imaging, computer systems, dataacquisition and analysis are the same as described above. This fiberoptic approach has all the advantages of the system described above aswell as the flexibility to independently move the measurement andreference fibers to any positions to suit the interferometricmeasurement configuration. If light source 12 comprises a shortcoherence length, the fiber lengths 68 and 72 must be adjusted so thatthe optical path lengths from variable fiber coupler 66 to the end offiber 72 for both the measurement beam 70 and reference beam 74 areequal.

Another embodiment of the invention is shown in FIG. 7. In thisembodiment light source 12 has a short coherence length and beams 18 and20 are reflected back through the polarization beamsplitter 22 so theyare coincident and collinear. Retroreflector 24 is positioned such thatoptical path length ACD is equal to optical path length ABDED. In otherword, retroreflector 24 is moved to a position where the round-trip beampath difference between the two retroreflectors is equal to theround-trip path between the interferometer plate and the optic undertest. The optical path lengths of the interfering measurement andreference beams are then the same. As in FIG. 1, PZT 30 can be attachedto either retroreflector. This condition produces high contrast fringesin the interference pattern and eliminates any extraneous interferencedue to light from the measurement beam spreading into the referencebeam. This configuration also eliminates any extraneous interference dueto light from the reference beam spreading into the measurement beam.

Another embodiment of the invention is shown in FIG. 8. This embodimentis the same as in FIG. 7, except that interferometer plate 36 isreplaced by single mode optical fiber 86. The far end of single modeoptical fiber 86 serves the same purpose as interferometer plate 36, andis shown in detail in FIG. 9. This embodiment is most useful whenmeasuring optics such that their conjugates coincide; for example, aswhen measuring a concave mirror at its center of curvature.

FIG. 9 shows the details at the far end of single mode optical fiber 86.It is embedded in substrate 88. The surface of the structure is polishedflat and smooth and coated with thin reflecting metal layer 90 that bothreflects and transmits approximately 40% of the light from the singlemode optical fiber 86.

Another embodiment of the invention is shown in FIG. 10. This embodimentis similar to FIG. 8 except that beams 18 and 20 are reflected backthrough polarization beamsplitter 22 so they are parallel to anddisplaced from each other. Reflected beam 18 now is reflected fromturning mirror 27, passes through polarizer 33 and is brought to a focuswith microscope objective 35, on a second single mode optical fiber 87.The measurement beam 89, leaving the end of single mode optical fiber87, is diffracted, producing a perfect spherical wavefront over somefinite solid angle. After passing through the optic 92 under test,aberrations are imparted to measurement beam 89. Focused measurementbeam 89 is reflected by reflecting metal layer 90 on the surface ofsingle mode optical fiber 86. It diverges and is coincident withreference beam 48 that is diffracted, producing a perfect sphericalwavefront over some finite solid angle. For interference to take placeas described previously, the length of single mode optical fibers 86 and87 are equal and retroreflector 24 is positioned such that the opticalpath length ACD is equal to optical path length ABF+GH. Anotherrequirement for interference to take place is that the polarization ofreference beam 48 and measurement beam 89 be identical. This isaccomplished by determining the polarization state of each beam, andthen physically manipulating the fibers until the measurement beam 89and the reference beam 48 have identical polarization states. Thisembodiment is most useful when measuring optics such that theirconjugates are spatially distinct, for example, as when measuring animaging system.

Although only those embodiments for the measurement of concave mirrorsand positive lenses were described, other embodiments exist formeasurement of convex mirrors and negative lenses

Changes and modifications in the specifically described embodiments canbe carried out without departing from the scope of the invention, whichis intended to be limited by the scope of the appended claims.

I claim:
 1. A phase shifting diffraction interferometer,comprising:means for separating a linearly polarized, collimated andcoherent beam of light into two parallel, spatially separated,orthogonally polarized coherent beams of light; means for introducing aphase shift between said two parallel, spatially separated, orthogonallypolarized coherent beams of light; a polarizer to orient said twoparallel, spatially separated, orthogonally polarized coherent beams oflight into two parallel, spatially separated, identically polarizedcoherent beams of light; means for focusing said two parallel, spatiallyseparated, identically polarized coherent beams of light to a focalpoint; an interferometer plate comprising:a glass substrate; a highlyreflective metallic film adherent to said glass substrate; and at leastone circular aperture through said glass substrate and said highlyreflective metallic film, wherein said circular aperture is placed atsaid focal point and has a diameter of about the size of the wavelengthof said coherent beam of light, wherein said interferometer platediffracts said two parallel, spatially separated, identically polarizedcoherent beams of light at said focal point to produce a measurementbeam and a reference beam; and means for focusing said measurement beamonto said aperture and said highly reflective metallic film, whereinsaid measurement beam and said reference beam combine to form aninterference pattern, wherein said means for separating a linearlypolarized, collimated and coherent beam of light into two parallel,spatially separated, orthogonally polarized coherent beams of lightcomprise: a light source for producing a linearly polarized, collimatedand coherent beam of light; a variable neutral density filter forcontrolling the intensity of said linearly polarized, collimated andcoherent beam of light; a half-wave retardation plate for producing,within said linearly polarized, collimated and coherent beam of light,two orthogonally polarized components of light comprising a verticalcomponent and a horizontal component, wherein the angular orientation ofsaid half-wave retardation plate is used to adjust relative intensitybetween said vertical component and said horizontal component; apolarization beamsplitter optically positioned to transmit saidhorizontal component to produce a first transmitted horizontallypolarized component, wherein said polarization beamsplitter is opticallypositioned to reflect said vertical component to produce a firstreflected vertically polarized component; a first retroreflectoroptically positioned to laterally reflect said first transmittedhorizontally polarized component back into said polarizationbeamsplitter for transmission therethrough to produce a horizontallypolarized beam; and a second retroreflector optically positioned tolaterally reflect said first reflected vertically polarized componentback into said polarization beamsplitter for reflection therefrom toproduce a vertically polarized beam; wherein said vertically polarizedbeam and said horizontally polarized beam together comprise twoparallel, spatially separated, orthogonally polarized coherent beams oflight, wherein said means for introducing a phase shift between said twoparallel, spatially separated, orthogonally polarized coherent beams oflight comprises a piezoelectric translator (PZT) mounted on aretroreflector selected from a group consisting of said firstretroreflector and said second retroreflector, wherein said PZTtranslates said retroreflector when a voltage is applied to said PZT,thus shifting the relative phase between said two parallel, spatiallyseparated, orthogonally polarized beams of light, wherein saidinterferometer plate further comprises a partially reflective metallicfilm adherent to said highly reflective metallic film, wherein saidpartially reflective metallic film covers said highly reflectivemetallic film and said aperture.
 2. The phase shifting diffractioninterferometer of claim 1, wherein said means for focusing saidmeasurement beam onto said aperture and said highly reflective metallicfilm comprises an optic to be tested.
 3. The phase shifting diffractioninterferometer of claim 2, further comprising an imaging system forimaging said interference pattern, said imaging system comprising aspatial filter comprising a first lens, an aperture and a second lens,wherein said spatial filter is positioned to transmit said interferencepattern, wherein said aperture is large enough that it does not diffractsaid interference pattern.
 4. The phase shifting diffractioninterferometer of claim 3, wherein said imaging system further comprisesa screen for displaying said interference pattern after it istransmitted through said spatial filter.
 5. The phase shiftingdiffraction interferometer of claim 3, wherein said imaging systemfurther comprises a charge coupled display camera and monitor fordisplaying said interference pattern after it is transmitted throughsaid spatial filter.
 6. The phase shifting diffraction interferometer ofclaim 3, further comprising a computer system having a centralprocessing unit, memory and software to: read said interference patternfrom said CCD camera, control the intensity and contrast of saidinterference pattern, translate said PZT, calculate the phase at eachpixel and display a resultant phase map.
 7. The phase shiftingdiffraction interferometer of claim 2, wherein said means for focusingsaid two parallel, spatially separated, identically polarized coherentbeams of light to a focal point comprises a prism in each beam of saidtwo beams, wherein said prisms refract said two beams away from eachother, said means for focusing said two parallel, spatially separated,identically polarized coherent beams of light further comprising a lens.8. A phase shifting diffraction interferometer, comprising:means forseparating a linearly polarized, collimated and coherent beam of lightinto two parallel, spatially separated, orthogonally polarized coherentbeams of light; means for introducing a phase shift between said twoparallel, spatially separated, orthogonally polarized coherent beams oflight; a polarizer to orient said two parallel, spatially separated,orthogonally polarized coherent beams of light into two parallel,spatially separated, identically polarized coherent beams of light;means for focusing said two parallel, spatially separated, identicallypolarized coherent beams of light to a focal point; an interferometerplate comprising:a glass substrate; a highly reflective metallic filmadherent to said glass substrate; and at least one circular aperturethrough said glass substrate and said highly reflective metallic film,wherein said circular aperture is placed at said focal point and has adiameter of about the size of the wavelength of said coherent beam oflight, wherein said interferometer plate diffracts said two parallel,spatially separated, identically polarized coherent beams of light atsaid focal point to produce a measurement beam and a reference beam; andmeans for focusing said measurement beam onto said aperture and saidhighly reflective metallic film, wherein said measurement beam and saidreference beam combine to form an interference pattern, wherein said atleast one circular aperture comprises two apertures, wherein said lensfocuses said two parallel, spatially separated, identically polarizedcoherent beams of light onto separate apertures of said two apertures.9. A phase shifting diffraction interferometer, comprising:means forproducing, within a collimated and spatially coherent beam of light, twoorthogonally polarized components of light comprising a verticalcomponent and a horizontal component; means for introducing a phaseshift between said vertical component and said horizontal component, toproduce a phase shifted beam; means for producing a measurement beam anda reference beam from said phase shifted beam; means for combining saidmeasurement beam and said reference beam to form an interferencepattern.
 10. The phase shifting diffraction interferometer of claim 9,wherein said means for producing, within a collimated and spatiallycoherent beam of light, two orthogonally polarized components of lightcomprising a vertical component and a horizontal component, comprise:alight source for producing a linearly polarized, collimated andspatially coherent beam of light; and a half-wave retardation plate forproducing, within said linearly polarized, collimated and spatiallycoherent beam of light, two orthogonally polarized components of lightcomprising a vertical component and a horizontal component, wherein theangular orientation of said half-wave retardation plate is used toadjust relative intensity between said vertical component and saidhorizontal component.
 11. The phase shifting diffraction interferometerof claim 10, wherein said means for introducing a phase shift betweensaid vertical component and said horizontal component comprise:apolarization beamsplitter optically positioned for reflecting saidvertical component to produce a first reflected vertically polarizedcomponent, wherein said polarization beamsplitter is opticallypositioned for transmitting said horizontal component to produce a firsttransmitted horizontally polarized component; a first retroreflectoroptically positioned to laterally reflect said first transmittedhorizontally polarized component back into said polarizationbeamsplitter for transmission therethrough to produce a horizontallypolarized beam; a second retroreflector optically positioned tolaterally reflect said first reflected vertically polarized componentback into said polarization beamsplitter for reflection therefrom toproduce a vertically polarized beam; wherein said vertically polarizedbeam and said horizontally polarized beam are colinear, and wherein saidmeans for introducing a phase shift comprise a piezoelectric translator(PZT) mounted on a PZT reflector selected from a group consisting ofsaid first retroreflector and said second retroreflector.
 12. The phaseshifting diffraction interferometer of claim 11, wherein said means forproducing a measurement beam and a reference beam from said phaseshifted beam comprise:a single mode fiber comprising an input end and anoutput end, wherein said output end is embedded in a substrate, saidoutput end further comprising a thin reflecting metal layer; means forfocusing said phase shifted beam into said input end of said single modefiber; wherein said first retroreflector and said second retroreflectorare positioned such that the round trip beam path difference betweensaid first retroreflector and said second retroreflector is equal to theround trip beam path between said output end of said single mode fiberand an optic under test.
 13. A phase shifting diffractioninterferometer, comprising:means for separating a linearly polarized,collimated and spatially coherent beam of light into two parallel,spatially separated, orthogonally polarized coherent beams of lightcomprising a vertically polarized beam and a horizontally polarizedbeam; means for introducing a phase shift between said verticallypolarized beam and said horizontally polarized beam; a first single modefiber comprising a first input end and a first output end, wherein saidfirst output end is embedded in a substrate, said first output endfurther comprising a thin reflecting metal layer; a second single modefiber comprising a second input end and a second output end; whereinsaid first single mode fiber and said second single mode fiber are thesame length, means for focussing said horizontally polarized beam intosaid first single mode fiber, wherein a reference beam diverges fromsaid first output end; means for focussing said vertically polarizedbeam into said second single mode fiber, wherein a signal beam divergesfrom said second output end; and means for combining said measurementbeam and said reference beam to form an interference pattern.
 14. Thephase shifting diffraction interferometer of claim 13, wherein saidmeans for introducing a phase shift between said vertically polarizedbeam and said horizontally polarized beam comprise a piezoelectrictranslator (PZT) mounted on a PZT reflector selected from a groupconsisting of said first retroreflector and said second retroreflector,wherein said PZT translates said PZT reflector when a voltage is appliedto said PZT, thus shifting the relative phase between said verticallypolarized beam and said horizontally polarized beam.